1. Field of the Invention
The present invention relates to a formation method of thin films, and a functional material and applied device therewith, in particular, to a formation method of transparent conducting oxide thin films for use in transparent electrodes, and a formation method of ferroelectric thin films for use in nonvolatile random access memory.
2. Description of the Related Art
A transparent conducting oxide thin films such as tin oxide (SnO2) or indium oxide (In2O3) have not only moderate conductivity, but also high transmittance in a visible wavelength range and excellent film strength as compared to metal thin films. For these reasons, such films are widely applied to, for example, electrodes for flat panel display. Further, a thin film composed of ferroelectric material such as lead zirconatea/lead titanate (PZT:Pb(Zr,Ti)O3) or strontium bismuth tantalate (SMT:SrBi2TA2O9) is expected as a new material to be applied to a nonvolatile memory with high rate, low power consumption and high accumulation.
A conventional formation method of such oxide thin films are classified into chemical film formation method and physical film formation method. The chemical film formation method includes spray, chemical vapor deposition, sol-gel method, coating or the like, where oxide is formed on a heated substrate by thermal decomposition of the compound such as chloride. Since these methods require a reaction temperature of usually 500xc2x0 C. or more, there are problems that materials for the substrate are limited, and that film properties are changed due to a material of the substrate.
On the contrary, as the physical film formation method, there are vacuum deposition, sputtering and the like. Such methods include a method in which metal or oxide, used as a starting material, is deposited on a heated substrate in an atmosphere of oxygen under a suitable pressure, another method in which a thin film composed of lower oxide or metal is formed on a substrate by vacuum deposition or sputtering, and then the formed film is oxidized by post-heat treatment, and the other method in which a deposited film which is formed in an atmosphere of prepurified oxygen gas is reduced by post-heat treatment. However, there is a problem that the formation of transparent conducting thin film requires the introduction of oxygen gas and substrate heating (about 300xc2x0 C. or more) to obtain low resistance and high transparency. Further, the formation of ferroelectric thin film requires the heating of 600xc2x0 C. or more in an atmosphere of oxygen to obtain a crystalline thin film with no oxygen deficiency.
On the other hand, a laser ablation method with low damage and non-thermal equilibrium processes is suitable for a formation of thin film of which properties such as contamination of impurity, crystallinity and surface condition. should be controlled. Further, in the laser ablation method, it is possible to form a thin film using various gases under a gas pressure condition with a wide range because of the transparency of laser beam. Since such characteristics do not depend on melting point and vapor pressure of material very much, the laser ablation method is expected to be applied to a formation of film of multicomponent material formed by processing (vaporizing and depositing) materials with different melting points and vapor pressures, which has been regarded as difficult to perform in a conventional thermal equilibrium process technology. For example, Japanese unexamined patent publication No. 5-270830 discloses that the laser ablation method is used in forming a superconducting oxide thin film and ceramic thin film. Further, Japanese unexamined patent publication No. 62-222058 discloses that the laser ablation method is used in forming a transparent conducting film.
However, materials ejected from a surface of oxide target by the laser ablation are vaporized with the target composition not held, and scattered as elements composing the target compound, whereby when the film formation is performed in a vacuum atmosphere, vaporized materials with lack of element having high vapor pressure (oxygen in this case) reach a substrate. For this reason, in the case where a thin film of indium tin oxide (ITO) is formed, fine particles of In or fine particles with excessive in are formed in the form of an island, with oxygen deficiency. Therefore, the thin film obtained by this method is inferior in transparency and conductivity. Further, in the case where a PZT thin film is formed using the laser ablation method, due to deficiency of Pb and O, capacity characteristics of the obtained thin film deteriorate. Such a problem should be solved to apply the laser ablation to the production of memory and the like. Because of it, a conventional case of a formation of oxide thin film using the above-mentioned laser ablation method requires the introduction of oxygen gas and the heating of substrate (about 300xc2x0 C. or more). When the heating of substrate is performed during the film formation in such a way, there is problem in a reactivity between materials used for in film formation and a substrate. Further, when oxygen gas is introduced in the laser ablation method, there is another problem that it is difficult to adjust the adaptability of an anaerobic process, for example, in semiconductor manufacturing processes.
An object of the present invention is to provide a thin film formation method capable of obtaining a thin film with the same composition as a target material easier than a conventional method.
This object is achieved by a thin film formation method comprising the steps of placing a target material and a substrate in a reaction chamber, adjusting a pressure (P) of an ambient gas to be introduced to the reaction chamber and a distance (D) between the substrate and the target material so as to optimize a size of an area with a high temperature and a high pressure formed at a place around the target material, and forming a thin film by irradiating a laser beam to the target material to be excited, while introducing the ambient gas into the reaction chamber under the pressure, and depositing species, contained in the target material, ejected from the target material on the substrate.
Thus, the present invention relates to a thin film formation method comprising placing a target material and a substrate in a reaction chamber; adjusting a pressure (P) of an ambient gas to be introduced to the reaction chamber and a distance (D) between the substrate and the target material so that crystal nucleus growth in a vapor phase is carried out in an area in which oxidization is promoted and which forms shock front caused by irradiating a laser beam to the target material; exciting the target material by irradiating the laser beam to the target material, while introducing the ambient gas into the reaction chamber under the pressure; and forming a thin film by depositing species, which are contained in the target material and ejected from the target material by being irradiated by the laser beam on the substrate.
Moreover, the present invention is directed to a thin film formation method comprising placing a substrate and a target material including at least two kinds of areas with different compositions in a target material in a reaction chamber; adjusting a pressure (P) of an ambient gas and a distance (D) between the substrate and the target material so that crystal nucleus growth in a vapor phase is carried out in an area in which oxidization is promoted and which forms shock front caused by irradiating a laser beam to the target material; exciting the target material by irradiating the laser beam to the target material, while introducing the ambient gas into the reaction chamber under the pressure; and forming a thin film by depositing species, which are contained in the target material and ejected from the target material by being irradiated by the laser beam, on the substrate.
Still further, the present invention is directed to an optoelectronic material obtainable by placing a substrate and a target material including at least two kinds of areas with different compositions in a target material in a reaction chamber; adjusting a pressure (P) of an ambient gas and a distance (D) between the substrate and the target material so that crystal nucleus growth in a vapor phase is carried out in an area in which oxidization is promoted and which forms shock front caused by irradiating a laser beam to the target material; exciting the target material by irradiating the laser beam to the target material, while introducing the ambient gas into the reaction chamber under the pressure; and forming a thin film by depositing species, which are contained in the target material and ejected from the target material by being irradiated by the laser beam, on the substrate.